Adam and Eve had sons and daughters(Genesis 5:4) Only three sons are mentioned though and they were Cain,Abel and Seth.
Where, then, did Cain get his wife? It was obviously from among his own family. His wife was one of his fleshly sisters, a daughter of Adam and Eve.“But such a union is incest!” some may object,It must be remembered, however, that circumstances were different in earlier times. Adam and Eve were created perfect, and the instruction for them ‘to multiply and fill the earth’ would necessarily require that their offspring marry one another and reproduce. (Gen. 1:28) But as perfect humans, their children would have been perfect as were their parents.
Even though Adam and Eve sinned and became imperfect, Cain and his brothers and sisters were still so near to physical perfection that the children they produced did not suffer the same adverse effects as do children born of such unions today. Even some 2,000 years afterward, God’s faithful servant Abraham married his half-sister Sarah, and God did not disapprove. (Gen. 20:12) It was yet another 450 years or so before God saw fit to provide his nation of Israel a body of laws that forbade incest on penalty of death. (Lev. 18:8-17) By that time imperfection had apparently developed to such an extent that no longer was it safe for close relatives to marry.
So, really, the question of where Cain got his wife is not a difficult one. The answer is provided right within the Bible, and it becomes obvious when the Bible account is read carefully.